Draft: chroot/base: Create chroot indicator file for use with boot-deploy

Marking as draft since boot-deploy does not yet implement customized chroot indicator filenames, and is currently hardcoded to /in-pmbootstrap. MR for that here: postmarketOS/boot-deploy!60 (merged)

Also marked as draft, because currently, /in-kupferbootstrap is not deleted in the final rootfs image. Maybe it can be bind mounted in chroot/helpers.py instead? Not sure if that would work.

Relevant comment thread: kupfer/packages/pkgbuilds!77 (comment 1870089440)

This would replace our current method of checking for the string kupfer in the booted kernel cmdline.